The hypothesis being tested is that thrombopoietic stimulation with Mpl ligands improves the effectiveness of platelet production sufficiently to normalize the megakaryocyte proliferation. This possibility was examined primarily in HIV-infected thrombocytopenic chimpanzees by administering the Mpl ligand pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF). The peripheral platelet counts increased 10-fold within 10 days without increasing viral loads or decreasing platelet antibody levels.